Obstruction to left ventricular outflow in hypertrophic cardiomyopathy (HC) is usually due to systolic anterior motion of the mitral valve. Analysis of 78 mitral valves in patients with HC disclosed that 13% had anomalous insertion of one or both left ventricular papillary muscles into mitral leaflet. Basal subaortic pressure gradients were large (70-150 mm Hg). In 8 patients, mitral valve replacement reduced the outflow gradient substantially (to 0-15 mm Hg). However, the other 2 patients who underwent septal myotomy-myectomy had persistent symptoms and incomplete relief of obstruction (gradients 60 and 70 mm Hg) due to continued mid-cavity apposition of papillary muscle and ventricular septum.